Heat pumps and refrigerators depend on conversion of one type of energy into heat energy in order to pump heat from a lower to a higher temperature.
The most commonly used system, the "vapor-compression cycle," relies upon the expansion and compression of a gas and utilizes the principle that, when a gas is compressed adiabatically, its temperature rises, and when it is expanded isenthalpically, its temperature diminishes. Typically, a fluid is employed to absorb heat from the compressed gas while another fluid gives up heat to the expanded gas.
Such mechanical systems dominate the cooling field today. These systems use liquid refrigerants that are typically chlorofluorocarbons. These refrigerants have been generally favored because they are easily volatilized, are inert to chemical reaction with most materials, are virtually odorless, nonflammable, noncorrosive, highly stable and have low toxicity compared to alternatives.
However, since 1974, chlorofluorocarbons have been the subject of environmental concern because of their ozone-destructive properties, culminating in an international treaty in 1989, the Montreal Protocol on Substances that Deplete the Ozone Layer, that established global limits on the production and use of chlorofluorocarbons. Under the United States Clean Air Act, chlorofluorocarbons are to be phased out by 1996. No estimate of the global cost of replacing chlorofluorocarbons technologies has been made, but it is estimated that in the United States alone, equipment based on these refrigerants is worth about $135 billion, equipment which must be modified or replaced. Thus, there is a need for new methods of heat transfer, refrigeration and heat exchange.
Furthermore, simple ventilation applications are also in need of energy efficient heat recovery systems that provide high quality indoor air. In recent times, a conflict exists between energy conservation measures and health factors arising from poor indoor air quality due to such conservation measures.
Alternative approaches to heat pumping, refrigeration, and the like are known, though none have approached the widespread use, efficiency and practical design of the vapor-compression systems. One such approach utilizes the magnetocaloric effect. Magnetocaloric systems utilize changes in magnetization to effect heat changes; certain magnetic materials warm upon magnetization and cool upon demagnetization. Various prototypes and models have been demonstrated. See, e.g., Pratt et al., Cryogenics, vol. 17 (1977) p. 689; Brown, J. Appl. Phys., vol. 47 (1976).
More recently, test results have been reported for an active magnetic refrigerator that uses the regenerator principle. See, e.g., U.S. Pat. No. 4,332,135 issued to Barclay et al.; A. J. DeGregoria et al., Adv. Cryogenic Eng. vol. 37, part B (1992) pp. 875-882. The regenerator principle involves heat recovery when a fluid (referred to as a shuttle fluid) is reciprocally exchanged between two reservoirs of different temperature, i.e., alternating flow by a hotter or colder fluid with some mechanism, such as the use of displacers, for effecting this reciprocating fluid flow through the system. The two-part regenerator cycle consists of flow of the fluid from the cold to the hot reservoir through a bed of porous heat transfer material, followed by flow of the fluid from the hot to the cold reservoir through the bed.
Where the heat capacity of the bed is very large compared to the heat capacity of the shuttle fluid, a temperature profile is established in the regenerator. The shuttle fluid is the total fluid mass that flows in one direction prior to reversal. After many reciprocating flows, the bed material establishes a temperature profile that increases from the side at which the cold fluid enters to the side at which the hot fluid enters. During the flow from cold to hot, the fluid enters at temperature T.sub.C, the temperature of the cold heat exchanger. It is warmed by the bed as it passes through the bed, and leaves the bed at a temperature below T.sub.H, the temperature of the hot exchanger. During flow from hot to cold, the fluid enters the bed at temperature T.sub.H. It is cooled by the bed as it passes through and leaves the bed at a temperature above T.sub.C. This difference in temperature of the fluid from entrance to exit from the bed, .DELTA.T, causes heat flow from the hot to cold reservoir. At worst, it is T.sub.H -T.sub.C, if there were no regenerator present. The ratio of .DELTA.T to (T.sub.H -T.sub.C) is referred to as the regenerator ineffectiveness. Over the cycle, the bed receives no net heat. It acts as an intermediate heat reservoir, absorbing heat from the warm gas and rejecting it to the cool gas.
Passive regenerative devices, for example, in the form of rotary air-to-air heat exchangers, have been described. U.S. Pat. No. 4,432,409 issued to Steele, describes a matrix (porous bed) formed of strips of plastic wound onto a hub with suitable spacing to form gas passages. Plastic is employed for its high heat capacity. U.S. Pat. No. 4,875,520 issued to Steele et al. describes a similar wheel arrangement in which a desiccant is applied to the plastic to make an enthalpy exchanger, i.e., a heat exchanger designed to remove both sensible heat and latent heat. Enthalpy exchangers offer significant advantages in many heat, ventilation and air conditioning (HVAC) applications since exchange of humidity as well as exchange of heat from indoors to outdoors can be minimized. In the Steele et al. device, a solvent is used to dissolve the outer layer of the plastic sheet before adding the desiccant particles. The desiccant particles are then added, partially embedding in the plastic.
In an active magnetic regenerator refrigerator, the porous bed is a magnetic material sandwiched between the two heat exchangers. Some mechanism exists for magnetizing and demagnetizing the bed. The cycle then consists of (i) bed magnetization, warming the magnetic material and bed fluid by the magnetocaloric effect; (ii) cold to hot fluid flow through the bed, transferring heat to the hot heat exchanger; (iii) bed demagnetization, cooling the magnetic material and fluid; (iv) and hot to cold fluid flow through the bed, absorbing heat at the cold heat exchanger. That is, the active magnetic regenerator magnetizes and warms the bed prior to fluid flow from cold to hot, then demagnetizing cools the bed prior to flow from hot to cold.
The single temperature profile of the bed in a passive regenerator now becomes a double profile for the active regenerator, one for the magnetized bed and one for the demagnetized bed. The difference between the two at any location is the adiabatic temperature change of the magnetic material in going through the field change. If the adiabatic temperature change is large enough, the fluid emerging from the cold end of the bed can have a temperature lower than T.sub.C, the temperature of the cold reservoir, resulting in net cooling, rather than a heat leak. According to the laws of Thermodynamics, of course, work must be done in the process since heat is flowing from a cold to a hot reservoir. In the case of a moving magnet, the work is performed by a drive.
Another principle that can be utilized in heat transfer/recovery systems is the thermoelastic effect. Certain elastomers, e.g., rubber, exhibit a thermoelastic effect in which the elastomer warms upon stretching and cools upon relaxing. Temperature changes as large as 14.degree. C. can occur, and for example, air (fluid) can be temperature-affected by forcing the air (fluid) over the elastomer as it is stretched and/or relaxed.
Elastomer refrigeration appears to permit more practical room temperature applications than magnetic refrigeration. In magnetic refrigeration, superconducting magnets are actually required to obtain adiabatic temperature changes large enough to be practical (approximately 8.degree. C. for a 7 Tesla magnetic field). These superconducting magnets require cryogenic refrigeration. Hence, only very large cooling power applications of magnetic refrigeration can be practical at room temperature. No such restriction obtains for elastomer refrigeration.
Some prior art devices utilize the thermoelastic effect; see, for example, U.S. Pat. No. 2,931,189 issued to Sigworth, U.S. Pat. No. 3,036,444 issued to Cochran, and U.S. Pat. No. 3,599,443 issued to Paine et al., all of which describe refrigerators, air conditioners and/or heat pumps using the thermoelastic effect of rubber. None of the disclosed designs, however, uses regeneration, and as a consequence, none of the devices can span a temperature greater than the adiabatic temperature change of the elastomer employed. Most of these designs also exhibit poor heat transfer between the elastomer and the fluid.
Thus, notwithstanding the many known problems with current mechanical systems and the practical design problems of alternative systems, the art has not adequately responded to date with an inexpensive, high performance device that can act both as a heat exchanger for ventilation and a heat pump for air conditioning or heating that utilizes regeneration and allows a high heat transfer. In particular, the art has not produced an elastomer heat exchanger that utilizes regeneration and allows a high heat transfer between the fluid and the elastomer for near room temperature regenerator applications.